Light guide for transmitting thermal radiation from melt to pyrometer and method of measuring temperature of molten metal in metallurgical vessel with the aid of said light guide

ABSTRACT

The light guide and the method of measuring temperature with the aid of said light guide relate to optical pyrometry of melts. 
     A light guide 1 is made from a light-permeable refractory corrosion-resistant material such as leucosapphire, for example, and according to the invention comprises 2 portions: a narrow portion 2 in the form of a rod, and a large portion 3 with a flat end 3a being an operating end of the light guide 1. The large portion 3 of the light guide 1 represents with respect to the narrow portion 2 an optical cavity producing radiation of the operating end 3a substantially in the form of a radiation of an absolutely black body. The ratio of the cross-section area of the narrow portion 2 at the place where it adjoins the large portion 3 does not exceed 0.5. 
     According to the method the light guide 1 is mounted in a lining 6 so that the large portion 3 is exposed to a melt 5 and the narrow portion 2 extends to a pyrometer 7 so that the geometrical axis of the light guide 1 passes through the point of the inner surface of the lining 6, located in a zone l of the maximum circulation of the melt 5. Said point is located with respect to a residual level 5a of the melt 5 at a depth h not less than the sum of a thickness t of the lining 6 at said level and a value equal to the half maximum transverse size d of the large portion 3 of the light guide 1. With the aid of the pyrometer 7 the temperature of the melt 5 is determined by measuring characteristics of the thermal radiation caused by the melt 5 in the area directly adjacent to said point on the inner surface of the lining 6 and transmitted to said pyrometer through the light guide 1. 
     The light guide and the method may substantially be employed in metallurgy and foundry for measuring temperatures of molten ferrous and non-ferrous metals.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to measuring techniques, in particular, tooptical pyrometry of molten metals and, more specifically, to a lightguide for transmitting thermal radiation from a melt to a pyrometer, andalso to a method of measuring temperature of a melt in a metallurgicalvessel with the use of said light guide.

2. Description of the Prior Art

Alongside with the unquestionable advantages the optical pyrometry hasover other methods used for measuring temperature, in particular, overthe measuring method using thermocouples immersed in a melt (apossibility to provide continuous temperature control, non-contactmeasuring process, etc.), the use of optical pyrometry for industrialproduction purposes is associated with certain difficulties. Inparticular, pyrometer readings depend to a large extent on opticalcharacteristics of the radiating surface and of the intermediate medium.

One of the most promising fields in optical pyrometry of molten metalsis the development of methods based on the use of light guides formingan isolated channel for transmitting thermal radiation from a melt to apyrometer thus allowing the influence of the abovementioned factors onmeasurement accuracy to be decreased.

An important problem which specialists in this field of opticalpyrometry are faced with is the development of light guides highlyreliable in operation, having satisfactory optical characteristics andproviding more accurate temperature measurements even in the case ofusing pyrometers which are simple in construction.

A great number of patents have been granted recently in variouscountries, which shows that this is an urgent problem and that attemptsare made to solve it (Cf., for example, the USSR Authors' CertificatesNo. 146,533, 1961, No. 271,067, 1970, U.S. Pat. No. 3,745,834, 1973, FRGPat. No. 2,338,532, 1976). In spite of the many attempts which have beenmade, the above problem has not been satisfactorily solved yet.

In all known constructions the light guide is arranged, for example, ina cylinder and has a constant cross-section. In particular, such is theembodiment of the light guide for transmitting/thermal radiation from amolten metal to a pyrometer disclosed in the Austrian Pat. No. 280,650,1970. This light guide is made from a light permeable refractorycorrosion-resistant material, for example, from quartz glass.

The temperature of the melt is measured with the aid of said light guidein the following manner. The light guide is mounted in the lining of anapparatus, preferably of a metallurgical vessel filled with a moltenmetal, so that the operating end of the light guide is in contact withthe melt. A spectral ratio pyrometer is positioned at the opposite endof the light guide. Thermal radiation caused by a high temperature of acontrolled medium (melt) is transmitted along the light guide throughthe metallurgical vessel lining to a pyrometer by whose readings atemperature of the melt is determined.

When measuring temperature of a melt with the aid of the above lightguide, the following difficulties arise. First of all it should be notedthat the influence of variation of radiation ability of the light guideoperating end on the measurement accuracy is not excluded. Thisvariation may be caused, for example, by a change in the chemicalcomposition of the controlled medium (change of a melt) or deteriorationof the operating end surface during its servicing (an increase of itsroughness, appearance of microcracks, etc.). In such cases one-valuedcorrespondence of the thermal radiation of the operating end to thetemperature of the latter and consequently the temperature of the meltis disturbed.

In addition, if the melt being controlled possesses such opticalcharacteristics that radiation ability of the light guide operating endand, consequently, the intensity of the radiation being transmitted areinconsiderable, there arises a necessity to use pyrometers with highlysensitive radiation receivers.

It should also be emphasized that to decrease the influence of thevariation in the radiation ability of the light guide operating end onthe measurement accuracy, as has been stated above, complex and costlypyrometers are required, such as a spectral ratio pyrometer and apyrometer with automatic correction. Nevertheless, even with the aid ofsuch pyrometers it is not possible to completely rule out the influenceof said factor on measurement accuracy.

It is also to be noted that in the prior art method of measuringtemperature of a molten metal with the aid of said light guide theproblem of selecting a best suited place for mounting the light guide inthe metallurgical vessel lining has not been solved. When the lightguide is mounted arbitrary in the lining, its operating end isfrequently deteriorated due to thermal shocks (sharp temperature change)occurring when a molten metal is poured into the vessel crucible orpoured out, when the furnace is charged or the vessel is inclined aswell as in other cases.

In addition to this, the light guide in many cases is deteriorated inthat part thereof which is set deep inside the lining, either due to thevariation of the temperature gradient with thickness of the lining inthe place where the light guide is mounted or due to the liningdisplacement with respect to the furnace shell and relative offsettingof said lining layers during operation.

It should be stressed that in so far as in the metallurgical vesselthere are zones wherein the temperature of the melt considerably variesfrom said vessel mass-average temperature, an arbitrary mounting of thelight guide will not ensure reliable information on its mass-averagetemperature. Let us also note that if the light guide is found in thecrucible slagging zone, then as a result of change in the temperaturedrop between the melt and the operating end of the light guide,measurement errors will be even greater.

Thus, an arbitrary mounting of the light guide does not provide arequired accuracy of measuring mass-average temperature of the melt inthe metallurgical vessel, which frequently does not meet the demands ofthe melting process.

SUMMARY OF THE INVENTION

The present invention resides in the provision of a method of measuringtemperature of a molten metal in a metallurgical vessel and a lightguide for transmitting thermal radiation for carrying out the same,which, by stabilizing characteristics of the thermal radiation beingtransmitted, would increase accuracy of measuring average masstemperature by using pyrometers of simpler designs.

These and other objects of the present invention are attained by a lightguide for transmitting thermal radiation from a molten metal to apyrometer, made from a light-permeable refractory corrosionresistantmaterial, which according to the invention, comprises a narrow part inthe form of a rod and a large part adjacent thereto having a flat endforming an operating end of the light guide, which large partrepresenting with respect to said narrow part an optical cavity used toform the radiation of the operating end, substantially, in the form ofradiation of an absolutely black body, with the cross-section area ratioof the narrow portion of the light guide at the place of the narrowportion adjoining the large portion to the area of the lateral surfaceof the large portion not exceeding 0.5.

The light guide of such construction simulates with its large portion anabsolutely black body whose thermal radiation, as is known, does notdepend either on the chemical composition of the simulating cavity orroughness of its inner surface but is solely determined by the absolutetemperature of this surface, which is derived from Kirchhoff's andPlanck's laws. In this case the narrow portion (the rod) of the lightguide performs the function of a channel for transmitting radiation ofsaid cavity through the metallurgic vessel wall to a pyrometer.Therefore when employing such a light guide, the radiation ability ofits operating end will practically be stable at a predeterminedtemperature of the molten metal irrespective of both its chemicalcomposition and the surface condition of this end, i.e. there willalways be observed an one-valued correspondence of the temperature ofthe operating end to its thermal radiation.

It should also be noted conformity with the Kirchhoff's law theradiation of heated bodies at the predetermined temperature isproportional to their absorbing ability. In so far as the absorbingability of the absolutely black body is maximal and is equal to 1, thisbody will radiate more energy than any other body having the sametemperature. That is why the employment of the proposed light guidesimulating an absolutely black body makes it possible to increase theintensity of radiation transmitted from a melt to a pyrometer to amaximum possible value at the predetermined temperature. Thisconsiderably reduces the requirements placed on the sensitivity of thepyrometers, their construction and ensures an increase of accuracy inmeasuring the temperature of the melt.

To attain the above result, in manufacturing the light guide it isnecessary to keep within the recommended ranges the area ratio of thecross-section of the narrow portion of the light guide to the area ofthe lateral surface of its large portion. In case this ratio exceeds0.5, the component of the measurement error arrising due to variation ofthe radiation ability of the light guide operating end is in excess of apermissible error in measuring the temperature of the melt.

The proposed light guide is suitable for mounting in any layer of themetallurgical vessel lining and is especially advantageous in the casewhen its large portion is located in the isothermic layer of the lining,i.e. in the layer with equal temperature for all the points coincidingwith the temperature of the melt where the light guide operating endcontacts the zone of said melt. In the isothermic layer between thelight guide operating end the lateral surface of its large portion isestablished an equilibrium thermal radiation ensuring most efficientobservance of an one-valued ratio between the temperature of theoperating end the radiation received by the pyrometer.

It is expedient that the proposed light guide be manufactured so thatthe lateral surface of its large portion have a mirror layer and such ashape that the radiation of the light guide operating end is reflectedfrom this surface back to the operating end thereof. The light guide ofsuch a modification can well be mounted both in the isothermic andnon-isothermic layers of the lining. In the latter case due to thecorresponding shape of the lateral surface of the light guide largeportion and a mirror coating thereof, all radiation of the operating endis reflected from said surface back to the same operating end. As aresult, the characteristics of the radiation passing through the narrowportion of the light guide to the pyrometer will always be constant atthe predetermined temperature of a melt.

In this case it is preferable to make the light guide in conformity withthe modification wherein the lateral surface of its large portion has asurface form of the body of revolution with a convex generatrix. As hasbeen proved by the research, such a shape of the light guide largeportion contributes to the greatest extent to reflecting its operatingend radiation back to the same end.

In particular, best results are achieved when the light guide largeportion is constructed in the form of a hemisphere whereas the narrowportion is constructed in the form of a cylindrical rod with a diameternot exceeding 0.8 the diameter of the hemisphere. If the rod diameterexceeds 0.8 the diameter of the hemisphere the temperature measurementerror is in excess of a permissible value.

These and other objects of the present invention are also attained bythat in a method for measuring the temperature of a molten metal in ametallurgical vessel, which comprises mounting a light guide in themetallurgical vessel lining and determining by means of a pyrometer thetemperature of a melt by its thermal radiation transmitted to thispyrometer by said light guide, according to the invention, the lightguide is placed so that its large portion is exposed to a melt and thenarrow portion of the light guide extends through the lining outside tothe pyrometer in such a manner that the light guide geometrical axispasses through the point of the lining inner surface, which point islocated in the zone of a maximum circulation of the melt and lyingrelative to the residual level of the melt at a depth not less than thesum of the lining thickness at said level and a value which is equal toa half the maximum transverse size of the light guide large portion, thetemperature of the molten metal being determined by measuringcharacteristics of the thermal radiation caused by the melt in the areaadjacent said point of the lining inner surface.

Here and further by the residual level of a melt is meant a level ofthat portion of the melt which remains in the crucible of themetallurgical vessel after melting has been finished and the main massof the molten metal has been poured out.

The proposed method realized with the aid of said light guide ensures ahigher measurement accuracy of the average mass temperature of themolten metal due to the following factors.

Firstly, in the zone of the greatest circulation of the melt practicallythere is no slagging, and a temperature drop between the main mass ofthe melt in this zone (and also the light guide operating end) will beminimum. In particular, for melting pig iron in an induction cruciblefurnace of a commercial frequency at a temperature of 1,500° C. and acirculation speed of up to 4 m/s this drop does not exceed 2K. That iswhy the point selected within the range of said zone is characteristicfor all the mass of the melt because said point has a temperatureessentially coinciding with the average mass temperature of the melt.Thus, if the characteristics of the thermal radiation are measured justin this point or on the portion adjoining it, the results of measuringthe temperature of the melt will be reliable.

Secondly, as has been proved by experiments a considerable variation inthe temperature gradient depending on the lining thickness, which maycause the light guide deterioration, takes place with respect to theresidual level of the melt within the depth of the melt which is equalto the lining thickness at this level. Therefore if said measuring pointselected within the zone of the greatest circulation of the melt islocated at a depth exceeding the lining thickness, then such a point isnot only a characteristic one but is also a point at whose level thetemperature gradient depending on the lining thickness varies to a smallvalue and only due to the change of the melt temperature in the processof the metallurgical vessel operation. For this purpose the light guidegeometrical axis must be located at a depth not less than the sum of thelining thickness at said level and the value equal to a half of themaximum transverse size of the light guide large portion. It is quiteunderstood that stability of the temperature gradient, which is achievedin this case prevents a failure of the middle portion of the lightguide. Furthermore, in so far as the light guide operating end,according to the proposed method, is constantly immersed in the melt(also when the main mass of the melt is poured out), thermal shockswhich cause deterioration of the operating end of said light guide areexcluded. All this ensures a stable transmission of the thermalradiation of the melt through the light guide to a pyrometer, whichimproves reliability and measurement accuracy.

It is expedient to realize the proposed method of measuring temperaturein accordance with the preferable embodiment thereof, wherein said lightguide is mounted in such a manner as to have said point on the innersurface of the lining at the closest possible distance within the rangeof the greatest circulation zone of the melt from the bottom of themetallurgical vessel. In this modification of the method embodiment aprobability of the light guide failure is further decreased because arelative offset of the lining layers which is one of the reasons of suchfailure is minimum at the bottom and increases with height of themetallurgical vessel.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be described further, by way of example withreference to the accompanying drawings, in which:

FIG. 1 is an axonometric representation of a light guide fortransmitting thermal radiation from a molten metal to a pyrometer,according to the invention;

FIG. 2 is a view similar to FIG. 1, but showing a modification, whereinthe lateral surface of the light guide large portion is coated with amirror layer and has the surface form of the body of revolution with aconvex generatrix according to the invention;

FIG. 3 is a view similar to FIG. 1, but showing a modification, whereinthe light guide large portion is constructed in the form of a hemispherewhile the narrow portion is constructed in the form of a cylindrical rodwith the light guide whole lateral surface being coated with a mirrorlayer, according to the invention;

FIG. 4 is a view similar to FIG. 1, but showing a modification, whereinthe light guide large portion is constructed in the form of a truncatedcone, according to the invention;

FIG. 5 is a view similar to FIG. 1, but showing a modification, whereinthe light guide large portion is constructed in the form of asemicylinder, according to the invention;

FIG. 6 is a view similar to FIG. 1 but showing a modification whereinthe light guide large portion is constructed in the form of a truncatedpyramid, according to the invention;

FIG. 7 is a view similar to FIG. 1 but showing a modification whereinthe light guide large portion is formed by a truncated cone and atruncated pyramid, according to the invention;

FIG. 8 is a partial cross-section of a metallurgical vessel with theproposed light guide mounted in the isothermic zone of the lining and apyrometer placed outside said metallurgical vessel (direction of thermalbeams is shown by arrows);

FIG. 9 is a view similar to FIG. 8, when the light guide is mounted inthe non-isothermic zone of the lining (radiated thermal beams are shownby solid arrows, reflected beams--by dotted arrows);

FIG. 10 is a diagrammatic representation of the embodiment for carryingout the method of measuring temperature with the aid or proposed lightguide (circulation of the melt in the metallurgical vessel is arbitraryshown by arrows); and

FIG. 11 is a view similar to FIG. 10 but showing a modification whereinfor the embodiment of said method the light guide is mounted close tothe bottom of the metallurgical vessel.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A proposed light guide 1 (FIG. 1) for transmitting thermal radiationfrom a molten metal to a pyrometer is made from a light-permeablerefractory corrosion-resistant material, of synthetic corundum, forexample. According to the invention, the light guide 1 has a narrowportion 2 in the form of a rod and adjoining thereto a large portion 3with a flat end 3a which is an operating end of said light guide. Thedimensions of the light guide 1 are selected so that the ratio of thecross-section surface of the narrow portion 2 at the place of the narrowportion adjoining the large portion 3 to the area of the lateral surface3b of the large portion 3 does not exceed 0.5. In this case thedetermining size is the cross-section area of the narrow portion 2, andmore accurately, the maximum transverse portion size which is selecteddepending on the strength characteristics of the material used for thelight guide 1 and the optical characteristics of the pyrometer for whichthe light guide is designed (the pyrometer is not shown in the drawing).

With such a ratio of said areas the large portion 3 of the light guide 1is with respect to the narrow portion 2 an optical cavity which makes itpossible to produce radiation of the operating end 3a arising when thelatter contacts the melt essentially in the form of radiation of anabsolutely black body. As has been shown above, at such a nature ofradiation of the operating end of the light guide 1 the radiationability of said end is stable at the predetermined temperature of themelt being controlled and does not depend either on the chemicalcomposition of the latter or on roughness of the operating end 3a.

It should also be noted that the shape of the cross-section of both thenarrow portion 2 and the large portion 3 of the light guide 1 may bedifferent and is determined depending on specific conditions of thelight guide employment such as the design of the metallurgical vessel,the location of mounting the light guide, the material it is made from,temperature variations of the melt and other factors. In this case saidconditions determine to a gereater extent the form of the large portion3 than the form of the narrow portion 2 of the light guide 1. In FIGS. 2through 7 of the accompanying drawings there are illustrated somealternate embodiments of the light guide 1 varying essentially in theform of the large portion 3 thereof.

FIG. 2 illustrates an embodiment, wherein the lateral surface 3b of thelarge portion 3 of the light guide 1 has the surface form of arevolution body with a convex generatrix. Said surface is coated with amirror layer 3c made from platinum, rhodium or some other materialsuitable for this purpose. The thickness of the layer 3c and itscomposition are chosen depending on the chemical composition of themelt, on the metallurgical vessel lining, their temperatures, dimensionsand material of the light guide as well as on the required time of thelight guide continuous operation. The narrow portion 2 of the lightguide 1 is constructed in the form of a rod having a rectangularcross-section.

With such a form, as shown in FIG. 2, of the lateral surface 3b of thelarge portion 3 of the light guide 1 the beams incident on said surfacefrom the operating end 3a will be reflected therefrom back to said endin conformity with the laws of geometrical optics. It is quite evidentthat in addition to the form of the body of revolution with a convexcomponent the lateral surface 3b may have some other form, wherein thethermal radiation of the operating end 3a is reflected from the surface3b back to the same end. Selecting some other similar form does notpresent a problem, and will be apparent to those skilled in the art. Themirror coating 3b with the corresponding form of the lateral surface 36makes it possible that the proposed light guide be well employed both inthe isothermic and non-isothermic layers of the lining.

The light guide in conformity with a modification illustrated in FIG. 2is better made from quartz glass and be employed in the metallurgicalvessels whose thickness of the isothermic layer of the crucible wall isless than the length of the large portion 3 of the light guide 1.

Here and further by the term "length" we shall designate the dimensionsof the light guide or its portion determined along the geometrical axisof the light guide.

The light guide constructed in accordance with said modification is fitfor mounting in the side wall of the metallurgical vessel, inparticular, in such metallurgical vessels as induction, channelfurnaces, crucibles, and bale-out pot furnaces.

Nevertheless, under the same conditions it is more profitable to employthe light guide constructed, as shown in FIG. 3 of the appendeddrawings. According to this modification the large portion 3 of thelight guide 1 is constructed in the form of a hemisphere whereas thenarrow portion 2 is constructed in the form of a cylindrical rod with adiameter not exceeding 0.8 diameter of the hemisphere. When the surface3b is of a spherical form, the number of reflections of thermal beamsradiated by the operating end when said end contacts the melt isminimal, which results in decreasing losses of the thermal energytransmitted by the light guide 1. In the given modification to furtherdecrease the energy losses, not only the large portion of the lightguide is coated with the mirror layer 3c but the narrow portion too iscoated with a similar layer 2a.

In the light guide, according to said modification, a cylindrical formthe narrow portion 2 has which is simpler for manufacturing than that inthe modification illustrated in FIG. 2. This is especially importantwhen for the production of the light guide hard-to-treat materials suchas synthetic corundum are used. When comparing possible uses of themodifications illustrated in FIGS. 2 and 3 it should be noted that theformer modification is more advantageous for metallurgical vessels,wherein the residual level of the melt is lower, whereas the lattermodification is more advantageous for metallurgical vessels wherein saidresidual level of the melt is higher.

In discussing modifications of the light guide given below (FIGS. 4through 7) we shall confine ourselves to describing the form of its mostessential large portion 3.

FIG. 4 of the appended drawings illustrates a modification wherein thelarge portion 3 of the light guide is constructed in the form of atruncated cone. In this case the larger base 3a of said cone is anoperating end of the light guide 1.

Such a modification of the light guide may be recommended formetallurgical vessels wherein thickness of the isothermic layer of thecrucible wall is not less than the length of the large portion 3 of thelight guide, which thickness allowing that said light guide be mounteddirectly at the bottom of a metallurgical vessel. In particular, thislight guide is especially suitable both for induction crucible furnacesand converters and can be made from synthetic corundum.

In the modifications illustrated in FIGS. 2 through 4 the geometricalaxes of both the large portion 3 and the narrow portion 2 coincide.Other modifications are also possible and, in particular, as shown inFIG. 5 of the accompanying drawings. According to this modification thelarge portion 3 is constructed in the form of a semicylinder whose axisis perpendicular to the axis of the narrow portion 2 of the light guide1, i.e. it is perpendicular to the optical axis of the latter.

When comparing the modifications illustrated in FIGS. 4 and 5 it shouldbe pointed out that the light guide 1, according to the latter of saidmodifications, possessing the same degree of blackness of the operatingend 3a, has a smaller longitudinal size of the large portion 3 than thatin the former modification. This enables the light guide illustrated inFIG. 5 to be employed for the metallurgical vessels having a thinnerisothermic layer, which makes it possible that this light guide shouldbe mounted both at the bottom and in the side wall of the metallurgicalvessel. Said modification of the light guide 1 may be recommended forsuch metallurgical vessels as induction furnaces, crucibles and bale-outpot furnaces when the light guide is made from quartz glass.

FIG. 6 of the appended drawings illustrates a modification of theproposed light guide wherein the large portion 3 is constructed in theform of a truncated pyramid. Such a modification of the light guide 1 issuitable for the metallurical vessels wherein thickness of theisothermic layer of the side wall is not less than the length of thelarge portion 3 when the light guide is made from quartz glass.

In particular, this light guide can well be employed in inductioncrucible, cupel, bale-out pot furnaces as well as in bath-furnaces andmagnetic hydrodynamic pumps.

It should be noted that the modifications of the light guide 1illustrated in FIGS. 2 and 3 are preferable for the metallurgicalvessels having wide fluctuations in the temperature of a melt, forexample, for high-frequency induction melting furnaces. Themodifications shown in FIGS. 4 through 6 are suitable for themetallurgical vessels having inconsiderable fluctuations in thetemperature of a melt such as cupolas, holding furnaces, bale-out potfurnaces, etc.

FIG. 7 of the appended drawings illustrates a modification of theproposed light guide wherein the large portion 3 is formed by atruncated cone 3d and a truncated pyramid 3e with their larger basesjoined. When having such a combined form of the light guide 1, it ispossible that the area of the operating end 3a be considerably reducedas compared to the area of the lateral surface 3b of the large portion 3and said area of the operating end 3a can practically be made verysmall. This, in turn, makes it possible that the influence of variationof the radiation ability of the operating end 3a on the measurementerror of the temperature of a melt be considerably decreased. Besides,the requirements are reduced as to thermal shock resistance of thematerials used for manufacturing the light guide 1 because the smalleris the operating end 3a contacting the melt the larger portion of thesurface of the light guide 1 can be protected against high temperaturesby a proper lining.

With a relatively small area of the operating end 3a, a probability ofdeterioration thereof from thermal shocks is reduced that is why thelight guide constructed as shown in FIG. 7 can be employed in suchmetallurgical vessels wherein the temperature of the melt varies in awide range. In particular, such wide fluctuation in the temperature ofthe melt occurs in converters, electric-arc furnaces and inductionmelting furnaces.

To better understand the nature of the present invention, refer to FIGS.8 and 9 of the accompanying drawings. Said Figures illustrate a bottomportion of a metallurigcal vessel 4 filled with melt 5 and the lightguide mounted in a lining 6 of the metallurgical vessel 4 and apyrometer 7 positioned behind the latter. In this case FIG. 8illustrates such an embodiment, wherein the large portion 3 of the lightguide 1 is completely located in an isothermic layer 6a of the lining 6,whereas FIG. 9 illustrates an embodiment wherein the large portion 3extends outside the boundaries of the isothermic layer 6a and, in fact,is placed in a non-isothermic layer 6b.

In the isothermic layer 6a of the lining 6 the temperature ispractically equal to all points and coincides with the temperature ofthe melt 5 in the zone wherein the operating end 3a of the light guide 1is located. That is why, if the large portion 3 of the light guide 1 iscompletely located in this layer, as shown in FIG. 8, between theoperating end 3a and its lateral surface 3b an equibalance thermalradiation is established. When such radiation is produced, both theoperating end 3a and the surface 3b are the sources of energy of equalintensity, each radiating as much energy per unit time as it absorbs.Such an exchange of thermal energy is diagrammatically illustrated bysolid arrows in FIG. 8. Through the narrow portion 2 of the light guide1 thermal radiation is delivered to the pyrometer 7. It should be notedthat in this case it is not obligatory to apply a mirror layer on thesurface 3o of the light guide 1 that is why the surface finish thereofmay be relatively not high.

In the case when the large portion 3 of the light guide is substantiallylocated in the non-isothermic layer 6b of the lining 6 (FIG. 9) only theoperating end 3a of the light guide 1 represents a radiator whereas thelateral surface 3o having a layer of the mirror coating 3c is areflector, i.e. it serves for reflecting thermal radiation backwards tothe operating end (reflected thermal beams are shown by dotted andradiated beams by solid arrows). Due to this the thermal radiation ofthe operating end 3a of the light guide 1, delivered to the pyrometer 7in spite of the temperature variations between said end and the surface3b, will be equibalanced, i.e. its characteristics will correspond tothe characteristics of radiation of an absolutely black body.

FIGS. 8 and 9 illustrate the process of the thermal radiation transferfrom the melt 5 to the pyrometer 7 with the aid of the light guide 1constructed according to a preferable embodiment similar to that shownin FIG. 3. But it is quite apparent that this process will proceedsimilarly if the light guide is constructed according to any othermodification discussed in detail above.

Thus, any of the described modifications of the light guide 1 (FIGS. 1through 7) ensures producing thermal radiation of the operating end 3asubstantially in the form of radiation of an absolutely black body,which makes it possible, as has been shown above, to considerablyincrease the intensity of this radiation and make it practically stableirrespective of roughness of the operating end and the chemicalcomposition of the melt 5 being controlled.

Measuring the temperature of the melt 5 with the aid of the light guide1 is carried on, according to the proposed method, as follows (FIG. 10of the appended drawings). The light guide is mounted in a lining 6 ofmetallurgical vessel, with a large portion 3 of said light guide facingmelt 5 and a narrow portion 2 thereof extending through the lining 6outside to a pyrometer 7. The light guide is mounted so that itsgeometrical axis passes through a strictly definite point on the innersurface of the lining 6.

Firstly, this point must be located in the zone of maximum circulationof the melt. The location of said zone and its boundaries may bedetermined with a sufficient accuracy for any metallurgical vessel. InFIG. 10 the zone of maximum circulation of the melt is confined by "l".Zones of slagging of the lining 6 are marked by reference numeral 8.

Secondly, said point must be located relative to the residual level 5aof the melt 5 at the depth "h" which should be not less than the sum ofthe thickness "t" of the lining at this level and a value which is equalto half the maximum transverse size "d" of the large portion 3 of thelight guide 1. Evidently, if the large portion 3 is constructed in theform of a body of revolution, said size is the maximum diameter of saidportion.

Thus, the depth "h" at which said point is located relative and residuallevel 5a of the melt 5 will be determined by:

    h>t+d/2

After mounting the light guide 1, the pyrometer 7 must be properlysighted on. For the purpose of measuring any of the known types ofpyrometers can be used including such relatively simple devices as apartial radiation pyrometer or a colour comparison monochromaticpyrometer whose constructions are known and require no detaileddescription.

Determining temperature of the melt 5 is performed with the aid of thepyrometer 7 by measuring characteristics of the thermal radiation causedby the melt 5 in the zone adjacent said point on the inner surface ofthe lining 6, and transmitted to said pyrometer by the light guide 1.

In such a method of measuring temperature, comprising the measuring at adefinite (characteristic) point whose coordinates are determineddepending on the characteristics of a metallurgical vessel (boundariesof the zone of maximum circulation of the melt, the column of theresidual level, thickness of the lining at this level), a high accuracyof measuring the mass-average temperature of the melt is achieved and areliability of the proposed light guide is increased.

A preferable embodiment of the proposed method and diagrammaticrepresentation thereof are illustrated in FIG. 11 of the accompanyingdrawings. According to this embodiment the light guide 1 is mounted sothat its geometrical axis passes through the point located within thezone "1" of the maximum circulation of the melt 5 at the minimumpossible distance from the bottom 4a of the metallurgical vessel 4. Insuch embodiment of the method the light guide is subjected to a minimummechanical load on the part of the lining 6 because close to the bottom4a a relative displacement of the lining 6 layers is insignificant. Thisconsiderably increases durability of the light guide and decreaseserrors in measuring the temperature of the melt 5.

The invention will now be described, with reference to the specificexamples of measuring temperatures of certain melts according to theproposed method and with the aid of the proposed light guide embodied invarious modifications.

EXAMPLE 1

For measuring the temperature of iron in a metallurgical vessel in therange from 1,300° to 1,500° C. a light guide made from leucosapphire wasused constructed as shown in FIG. 3, and having a mirror coating on thewhole of its lateral surface. Measurements were carried out according tothe proposed method with the aid of a colour comparison monochromaticpyrometer with the operating wave length equal to 0.65μ. In this casethe parameters of the light guide and the metallurgical vessel were asfollows:

the total length of the light guide--200 mm;

the maximum transverse size of the light guide large portion (diameterof the hemisphere)--40 mm;

the ratio of the diameter of the light guide narrow portion to that ofits large portion d₁ /d₂ -- 0.2;

the lining thickness of the metallurgical vessel at the levelcorresponding to the residual level of the melt--150 mm;

thickness of the isothermic layer of the lining--not exceeding 10 mm.

With this stated thickness of the lining and its isothermic layer thelarge portion of the light guide was embedded, in fact, in thenon-isothermic layer of the lining.

In this case the absolute change in the radiation ability of the lightguide operating end caused by the formation of the oxide film on saidend did not exceed 0.01 which is 35 times less than in the prior artlight guide under the same conditions. The component of the measurementerror corresponding to said change of the radiation ability did notexceed 0.1% whereas in the prior art light guide this component reached3.5%.

EXAMPLE 2

Under the conditions similar to example 1 a light guide was used havingthe ratio of d₁ /d₂ equal to 0.5. In this case the component of themeasurement error did not exceed 0.44% which is 8 times less than in theprior art light guide.

EXAMPLE 3

Under the conditions similar to example 1 a light guide was used havingthe ratio of d₁ /d₂ equal to 0.8. In this case the component of themeasurement error did not exceed 1.1% which is more than 3 times lessthan in the prior art light guide.

EXAMPLE 4

For measuring the temperature of molten quartz glass in the range from1,800° to 1,900° C. a light guide was used constructed similarly to thatillustrated in example 1. Measurements were carried out by the partialradiation pyrometer with the operating spectral range from 0.8 to 1.8μ.

In this case the absolute change in the radiation ability of the lightguide operating and, caused by the roughness alteration of said end inthe process of operation, did not exceed 0.004 which is 45 times lessthan <-> light guide <in the prior art> under similar conditions. Inthis case the component of the measurement error corresponding to saidvariation of the radiation ability did not exceed 0.4% whereas <in theprior art> light guide this component reached 18%.

EXAMPLE 5

Under the conditions similar to example 4 a light guide was used havingthe ratio of d₁ /d₂ equal to 0.3. In this case the component of themeasurement error did not exceed 0.8% which is more than 20 times lessthan in the prior art light guide.

EXAMPLE 6

Under the conditions similar to example 4 a light guide was used havingthe ratio of d₁ /d₂ equal to 0.4. In this case the component of themeasurement error did not exceed 1% which is 16 times less than in thelight guide prior art.

EXAMPLE 7

With the aid of the same light guide and in the same metallurgicalvessel, shown in example 1, temperature of molten silver was measured inthe range from 1.100° to 1.200° C.

In this case the absolute variation of the radiation ability of thelight guide operating end caused by the formation of the oxide film onsaid end did not exceed 0.02 which is 45 times less than in the priorart light guide under similar conditions. In this case the component ofthe measurement error corresponding to said variation of the radiationability did not exceed 0.2% whereas in prior art the light guide thiscomponent reached 9%.

EXAMPLE 8

Under the conditions similar to example 7 a light guide was used withthe ratio of d₁ /d₂ equal to 0.3. In this case the component of theerror measurement did not exceed 0.4% which is more than 22 times lessthan in the prior art light guide.

EXAMPLE 9

With the aid of the same light guide and in the same metallurgicalvessel, shown in example 1, the temperature of molten copper wasmeasured in the range from 1,200° to 1,300° C.

In this case the absolute variation of the radiation ability of thelight guide operating end caused by the formation of the oxide film onsaid end did not exceed 0.01 which is 50 times less than in the priorart light guide under similar conditions. In this case the component ofthe measurement error corresponding to said variation of the radiationability did not exceed 0.1% whereas in the prior art light guide thiscomponent reached 5%.

EXAMPLE 10

Under the conditions similar to example 9 a light guide was used withthe ratio of d₁ /d₂ equal to 0.3. In this case the component of themeasurement error did not exceed 0.3% which is 17 times less than in theprior art light guide.

EXAMPLE 11

Under the conditions similar to example 1 a light guide was usedconstructed as shown in FIG. 2, with the ratio of the area of the narrowportion cross-section of said light guide to the area of the lateralsurface of its large portion s₁ /s₂ equal to 0.2. In this case thecomponent of the measurement error did not exceed 1.1% which is 3.5times less than in the light guide prior known in the art.

EXAMPLE 12

Under the conditions similar to example 4 a light guide was used, asshown in FIG. 2, with the ratio of s₁ /s₂ equal to 0.06. In this casethe component of the measurement error did not exceed 1% wich is 18times less than in the light guide prior known in the art.

EXAMPLE 13

Under the conditions similar to example 1 a light guide was usedconstructed, as shown in FIG. 7, with the ratio of s₁ /s₂ equal to 0.5.The light guide was mounted so that its large portion was completelyembedded in the isothermic layer of the linging. In this case thecomponent of the measurement error did not exceed 1.7% which is 20 timesless than in the light guide prior known in the art.

EXAMPLE 14

Under the conditions similar to example 4 a light guide was usedconstructed, as shown in FIG. 7, with the ratio of s₁ /s₂ equal to 0.05.The large portion of the light guide was completely embedded in theisothermic layer of the lining. In this case the component of themeasurement error did not exceed 0.8% which is 20 times less than in thelight guide prior known in the art.

EXAMPLE 15

Under the conditions similar to example 1 a light guide was usedconstructed, as shown in FIG. 4, with the ratio of s₁ /s₂ equal to 0.25.The large portion of the light guide was completely embedded in theisothermic layer of the lining. In this case the component of themeasurement error did not exceed 1.3% which is more than 2 times lessthan in the prior art light guide.

EXAMPLE 16

Under the conditions similar to example 4 a light guide was usedconstructed, as shown in FIG. 4, with the ratio of s₁ /s₂ equal to 0.08.The large portion of the light guide was completely embedded in theisothermic layer of the lining. In this case the component of themeasurement error was about 1% which is 18 times less than in the priorart light guide.

EXAMPLE 17

For measuring the temperature of molten aluminium in the range from 650°to 800° C. a light guide made from quartz glass was used constructed, asshown in FIG. 5, having the ratio of s₁ /s₂ equal to 0.1 as well as thepartial radiation pyrometer identical to that in example 4. The totallength of the light guide and the lining thickness were identical tothose stated in example 1.

In this case the absolute variation of the radiation ability of thelight guide operating end stipulated by alteration of its roughness inthe process of operation thereof did not exceed 0.01 which is 10 timesless than in the light guide prior known in the art under similarconditions. In this case the component of the measurement errorcorresponding to said variation of the radiation ability did not exceed1% whereas in the prior art light guide this component reached 10%.

EXAMPLE 18

Under the conditions similar to example 17 a light guide was usedconstructed, as shown in FIG. 6, whose ratio s.sub. /s₂ was equal to0.04.

In this case the component of the measurement error did not exceed 0.5%which is 20 times less than in the prior art light guide.

EXAMPLE 19

In order to determine the influence of the light guide mounting site onthe reliability of its operation, 3 groups of light guides, 15 pieces ineach, were mounted in the induction furnace for melting pig iron. Allsaid light guides were made from leucosapphire.

The light guides of the first and second groups were used for measuringthe temperature of the molten metal according to the proposed method andconstructed, as shown in FIG. 4. In this case each light guide of thefirst group was mounted as shown in FIG. 10 and each light guide of thesecond group was mounted as shown in FIG. 11.

The third group included known in the art light guides, in particular,having the form of a cylindrical rod with the aid of which measurementswere taken according to the known in the art method, i.e. thedisposition of these light guides did not correspond to that shown inFIGS. 10 and 11. In particular, the first 5 light guides were positionedhigher than the residual level of the melt, the next 5 light guides werepositioned outside the zone of the maximum circulation of the melt andeach of the remainder light guides was positioned relative to theresidual level so that its geometrical axis was at a depth smaller thanthe lining thickness at this level.

For 1,500 hours of the furnace operation (the lining thermal life) onlyone light guide (FIG. 10) was deteriorated in the first group while inthe second group (FIG. 11) no deteriorated light guides were observed atall. In this case the measurement error for the light guides of thefirst and second groups was within the limits of a permissible value.

At the same time all the light guides of the third group were destroyedand the measurement error when employing these light guides greatlyexceeded a permissible value. Such a great mesurement error wasstipulated, in particular, for the first 5 light guides by thedeterioration of the operating ends and the middle portions of saidlight guides, for the next 5 light guides by the deterioration of theirmiddle portions and for the remainder 5 light guides by thedeterioration of their middle portions and slagging of the operatingends thereof.

EXAMPLE 20 (negative)

Under the conditions similar to example 1 a light guide was used, asshown in FIG. 4, whose ratio s₁ /s₂ was equal to 0.75, i.e. exceeded aspecified value. The light guide large portion was completely embeddedin the isothermic layer of the lining.

In this case the component of the measurement error reached about 4%,which considerably exceeds a permissible value of the total measurementerror.

EXAMPLE 21 (negative)

Under the conditions similar to example 4 a light guide was used, asshown in FIG. 3, wherein the ratio of d₁ /d₂ was equal to 0.9, i.e.exceeded a specified value. The light guide large portion was embedded,in fact, in the non-isothermic layer of the lining.

In this case the component of the measurement error was equal to 8%which considerably exceeds a permissible value of the total measurementerror.

While particular embodiments of the invention have been shown anddescribed, various modifications thereof will be apparent to thoseskilled in the art and therefore it is not intended that the inventionbe limited to the disclosed embodiments or to the details thereof andthe departures may be made therefrom within the spirit and scope of theinvention as defined in the claims.

Most efficiently the present invention can be used in metal smelting andcasting for measuring temperature of ferrous and non-ferrous metals insuch metallurgical vessels as induction furnaces, open-hearth furnaces,converters, magnetic hydrodynamic pumps, etc. Moreover, it can also beused in glass and chemical industries for measuring temperature ofmolten glass, solt, and other materials.

We claim:
 1. A light guide for transmitting thermal radiation from amelt to a pyrometer, being substantially a rod made from a refractorylight-permeable corrosion-resistant material having a narrow portionintended for transmitting radiation to the pyrometer and a large portionwith a flat end forming the operating end of the light guide, thelateral surface of said large portion providing reflection of radiationof the operating end of said large portion from the convex lateralsurface of this portion back to said flat operating end; the ratio ofthe cross-sectional area of said light guide narrow portion at the placeof adjoining said large portion to the lateral surface area of thislarge portion not exceeding 0.5.
 2. A light guide as set forth in claim1, in which the lateral surface of the large portion of the light guideis coated with a mirror layer.
 3. A light guide as set forth in claim 1,in which the large portion thereof is constructed in the form of ahemisphere, and the narrow portion is in the form of a cylindrical rodhaving a diameter not exceeding 0.8 the diameter of the hemisphere.
 4. Alight guide as set forth in claim 1, wherein said lateral surface ofsaid large portion having the form of a surface of a body of revolutionwith a convex generating line.
 5. A method of measuring the temperatureof a melt in a metallurgical vessel with the use of the light guidehaving a narrow portion intended for transmitting thermal radiation fromthe melt to the pyrometer and a large portion with a flat end formingthe operating end of the light guide, comprising the steps of mountingthe light guide so that its large portion is exposed to the melt and thenarrow portion is adjoined through the lining outside to the pyrometerso that the geometrical axis of the light guide passes through the pointof the inner surface of the lining, located in the zone of the maximumcirculation of the melt and lying with respect to the residual level ofthe melt at a depth h not less than the sum of the thickness t of thelining at said level and a value equal to half the maximum transversesize d of the large portion of the light guide, with the temperature ofthe melt being determined by measuring characteristics of the thermalradiation of the operating end which is in contact with the melt at saidpoint.
 6. A method as set forth in claim 5, in which the light guide ismounted so that said point on the inner surface of the lining ispositioned within the zone of the maximum circulation of the melt at theminimum possible distance from the bottom of the metallurgical vessel.7. A method for measuring the temperature of the melt in a metallurgicalapparatus with the use of the light guide having a narrow portionintended for transmitting thermal radiation from the melt to thepyrometer and a large portion with a flat end forming the operating endof the lightguide, comprising arranging the lightguide in the lining ofthe apparatus and measuring the temperature of the melt with the help ofa pyrometer using thermal radiaion transmitted through the lining bysaid narrow portion of the lightguide, characterized in that thelightguide is arranged so that its wide portion is in contact with themelt, and the narrow portion of the lightguide is extended through thelining outside to the pyrometer in the area of the lining which islocated relative to the residual level of the melt at a depth equal toat least the thickness of the lining in contact with the lightguide, thelightguide being arranged at a minimum possible distance from the bottomof the apparatus within the melt circulation zone.